To operate an auto-ignition combustion engine such as a diesel engine, for instance, a combustion-position control is carried out for new combustion processes, by which the points of injection of fuel into the cylinders are set. Such a control is based on pressure features ascertained in the combustion chamber, such as data about a combustion position or the indicated work, in particular, based on the combustion feature MFB50% (mass fraction burned) (MFBx %: crankshaft angle at which x % of the fuel injected in the cylinder is burned).
With the control of the combustion position, the combustion in the cylinders is suitably corrected as a function of the control deviation by at least an intervention with respect to the point of injection, in order to achieve a desired combustion which is determined by a setpoint value of the combustion feature. Since, in principle, such a control is only able to react after the control deviation has occurred, and the speed with which the controller can be applied must be limited for reasons of stability, the setting to the desired combustion is only possible with a delay which, according to experience, corresponds to several working cycles of the combustion engine.
In the case of new diesel combustion systems, which are implemented to reduce exhaust-gas emissions, such as LTC (Low Temperature Combustion), HCCI (Homogeneous Charge Compression Ignition), HCLI (Homogeneous Charge Late Ignition), generally very high exhaust-gas recirculation rates (EGR-rates) are provided. Given a corresponding engine system, the EGR-rate determines the portion of the quantity of combustion exhaust gas returned into an air-feed system relative to the total amount of the gas mixture directed into the cylinders.
Because of the high EGR rates used in the new diesel combustion systems, the combustion engine is operated closer to a stability limit, so that in response to an EGR-rate deviating slightly from the setpoint EGR-rate set, the risk of an unwanted operating state of the combustion engine develops. The exceeding of the optimal EGR-rate leads, in particular, to an increased risk of misfirings, and if there is a drop below the optimal EGR-rate, the noise generation during the combustion increases unacceptably. Especially during dynamic operation with respect to the speed and the load of the combustion engine, in which, because of the sluggishness of the air system, it is inherent in the system that the necessary EGR-rate is exceeded or not attained, this represents a challenge for the engine management.
Because of the delay described above, the conventional control of the air system is not able to continuously retain the relevant air-system variables, such as the EGR-rate, at the optimal value. A combustion control, and here especially the control of the combustion position to the steady setpoint value for the combustion position MFB50% in dynamic operation, is not practical due to the different state of the air system, since here a different setpoint value represents the optimum. Therefore, the dynamic operation of the combustion engine may be associated in particular either with too high a noise generation or with an increase in the risk of combustion misfirings.
In addition, the engine speed has an influence on the combustion, since, among other things, it influences the mixture preparation. Since, for stability reasons, the speeds with which the controls can be applied must be limited, in the case of longer deviations between setpoint and actual values, a not inconsiderable integration component may build up which may lead to misfirings in the event of rapid changes in the operating point and a rapid commencement of the exhaust-gas recirculation as a result.
Therefore, an object of the exemplary embodiments and/or exemplary methods of the present invention is to provide a method and a device with which it is possible to compensate for the effects of the deviation of the EGR-rate from the optimal EGR-rate, or in general, a deviation of the air-system variables from their setpoint values, particularly during dynamic operation of the combustion engine, as quickly as possible via the rapid control path of the fuel injection, and in particular, to minimize or rule out the occurrence of disadvantageous operating states because of the condition of the air mixture upstream of the intake valves, e.g., a drop below or exceeding of the EGR-rate and/or the mixture temperature.